Is it obligatory for me to appraise the land I purchased for a price lower than its market value and pay the difference to the seller, taking into consideration my current financial situation?

The default is that you are not obligated to do that. Your mere assumption that the person is foolish or mentally deficient, or that the intermediary defrauded him, does not entail anything. This is because the default for contracting parties is soundness, and their actions are presumed to be valid unless proven otherwise.